Multiple quantum wells are semiconductor structures comprised of alternating thin layers of two different semiconductor materials and, in particular, of semiconductor materials having differing bandgaps. Typically, layer thicknesses are of the order of 100 Angstroms and a typical structure might comprise 100 such layers, resulting in a total thickness of about 1 micrometer. Multiple quantum well structures are typically produced using well known epitaxy techniques, such as molecular beam epitaxy or metal-organic chemical vapor deposition sometimes known as organometallic vapor phase epitaxy. Multiple quantum well structures have been used successfully in many different optical devices, such as optical modulators.
The quantum confined Stark effect (QCSE) has given rise to several innovations in electro-optic modulators. Such modulators have many applications in communications and special purpose computer systems. The principals behind the QCSE have been more fully explained by D. A. B. Miller et al, in Physics Review, 1985, B32, p1043. Briefly though, the QCSE is a phenomenon which arises when an electric field is applied across the plane of heterostructure superlattices. In a quantum well at zero electric field, the electron and hole energy levels are defined by the well width, and the electrons and holes are strongly confined in the well layer. However, when an electric field is applied, the electrons and holes are moved apart and their energies are altered. This has the effect of shifting the absorption resonance to lower energy as well as modulating the strength of the absorption. This occurs because direct optical absorption of a photon above the band gap energy involves raising an electron from one of the valence bands and putting it in the conduction band, otherwise known as formation of an electron-hole pair. This shift in the absorption resonance, then, provides for the optical modulation of any radiation that is incident to the heterostructure.
A typical structure for such an optical modulator, also known as a quantum confined Stark effect (QCSE) modulator, is a p-i-n diode with the multiple quantum well structure formed within the intrinsic layer of the diode, i.e., the "i" region. In operation, a light beam is either directed perpendicular to the multiple quantum well layers or in the plane of the layers in a waveguide configuration, while at the same time a reverse bias is applied to the diode. Modulation of the light beam is effected by varying the reverse bias. An example of such a device is found in U.S. Pat. No. 5,105,301, issued on Apr. 14, 1992 to Campi and entitled, "Coupled Quantum Well Electro-optical Modulator."
Examples of both electrically controlled and optically controlled multiple quantum well devices can also be found in an article by D. A. B. Miller, "Optoelectronic applications of quantum wells," Optics Photonics, vol 1, no. 2, page 7, Feb. 1990; U.S. Pat. No. 4,546,244, issued in October, 1985 to Miller; and U.S. Pat. No. 4,904,859, issued on Feb. 27, 1990 to Goossen et al and entitled, Self Electro-optic Effect Device Employing Asymmetric Quantum Wells."
In other types of devices, waveguides are formed side by side in a common plane with a suitable coupling material between. Typically, the index of refraction in the two waveguides is identical and that of the coupling material is lower so that there is resonant coupling between the waveguides in a cross propagation or "switch" condition. A "no switch" or parallel propagation condition is commonly created by an induced change in one of the guides, normally caused by an electric field induced change of the index of refraction (dn/dE) in one waveguide. The value of dn/dE is a factor which determines the magnitude of the electric field required to switch the light. The larger dn/dE, the smaller the voltage required for a given geometric configuration. Other factors affecting the operation of such devices are the index of refraction and width of the coupling material. Within limits, the smaller the difference between the indices of refraction of the waveguides and the coupling material, and the narrower the width of the coupling material, the shorter is the length of the parallel waveguides required for cross coupling. In turn, a shorter device length results in decreased device capacitance, and thus an increase in the maximum switching speed and a decrease of the energy required per switching cycle.
One such electro-optic coupler is described in U.S. Pat. No. 4,923,264 issued to Langer et al on May 8, 1990, which is incorporated by reference hereto. This patent describes an electro-optic coupler which is made of consecutively deposited layers of semiconductor material including a waveguide layer having a specific index of refraction, a second waveguide layer mode of an electro-optically active multiple quantum well structure having a second index of refraction, and a coupling layer between the waveguide layers having a third index of refraction. In operation, an electric field is applied across the coupler and the electric field affects the index of refraction of the multiple quantum well structure to make it equal to the index of refraction of the first waveguide. Therefore, the signal may be coupled from one waveguide to the other or the signal may be confined to the first waveguide.
However, all the aforementioned couplers, modulators and/or switches lack the ability to switch a signal in any one of four directions.